the accln may defined as the change (final - initial, i.e the change here) in velocity devided by the time. construct the eqn. and put initial velocity as zero, get an eqn with time, t and final velocity, v, as two unknown quantities. now get another eqn. equating the initial energy of the boy, potential energy here, and final energy( kinetic energy). it follows from conservation of energy concept. thus u can get an eqn with only one unknown, v. the rest is easy.